P/1789 USA

The Dissociation of Diatomic Hydrogen Ions

By С F. Barnett*

The thermonuclear effort at the Oak Ridge NationalLaboratory has as a primary interest the developmentof methods for the trapping of a deuteron beam in acontainer employing magnetic mirrors. Luce1» 2 hasproposed that deuterons could be placed in a stableorbit by dissociating diatomic ions with a concentratedgaseous discharge in the form of a high-current arc.With a good efficiency of dissociation of the diatomicions, an energetic plasma may be formed provided theenergy of the trapped particles is large enough toinsure that the electron capture cross section is as lowas about 10~19 cm2, and provided that the neutralparticle density in the background gas can be reducedin accordance with the possibilities of modern vacuumtechnique. The use of dissociation for orbital injectionhas also been independently proposed for high-energyaccelerators,3 although the use of an arc as an agentfor the dissociation does not appear to have beencontemplated in that connection.

The promise of such a proposal and the choice of thebest energy for the injection of the ions into the trap-ping geometry depend upon certain interaction crosssections of T>2+ ions which heretofore have not beenmeasured.4 General information on the behaviorof the D2+ dissociation cross section by collisionsin various gases is of physical interest, but more directimportance attaches to measurement of the probabilitythat a D2+ ion be dissociated in passage through an arccolumn, and it is also important to know the pro-bability that a D + ion capture an electron in passingthrough the arc, because this represents a loss mech-anism for the trapped beam. In this paper we describethe measurement of these quantities in the energyrange 20 kev to 2.25 Mev. H24" ions were used ratherthan D2

+, under the assumption that the cross sectionswill be equal at equal velocities. The first part of thepaper describes the measurement of Нг+ dissociationcross sections, and the second part describes theexperiments in which the ion beams were shot throughthe column of a 300-ampere vacuum carbon arc.

DISSOCIATION BY IMPACT WITH GAS ATOMS

The dissociation cross section of Нг+ in collisionswith gases has been studied theoretically by Salpeter5

using the Born approximation. Results are given for

* Oak Ridge National Laboratory, operated by UnionCarbide Corporation for the US Atomic Energy Commission.

398

energies in excess of 2 Mev, which is the energy regionwherein the Born approximation might be expectedto be valid. The calculation indicates that dissociationis largely produced by excitation of the covalentelectron into a repulsive state, and the cross section ispredicted to vary inversely as the square of the par-ticle velocity. Gerjuoy6 has treated the problem ofdissociation by impact of Нг+ with protons in theenergy interval 10 to 500 kev. The cross section showsa maximum at 100 kev, but thereafter decreases alsoas 1/Л

The dissociation cross section was first measured byEffat7 for energies of 9 and 18 Mev. More recently,Federenko8 has measured the cross section at lowenergies (5-30 kev) and Damodaran9 has studied theenergy interval 100-200 kev.

Previous papers from this Laboratory10"12 havedescribed the apparatus used here for the determina-tion of atomic collision cross sections, and inasmuch asthe same equipment served for the measurements ofmolecular ion dissociation, a detailed description willnot be necessary. Suffice it to say that, as is customaryin such experiments, a H24" beam of adjustable energywas passed through apertures at the ends of a differen-tially-pumped gas cell, and the emergent beam wasanalyzed to give the currents of H+, H24" and neutralspecies.

In the low energy range (20-200 kev) of incidentH24" ions, the measurements present some difficulties,one of which arises from the fact that the neutralcomponents H20 and H° are inseparable with ordinarydetectors. Under such circumstances, it is usually bestto take the sum of the emergent charged beamcurrents, H+ and H2+ as an approximation to theincident beam current, thereby neglecting the fractionof the beam that undergoes simple electron captureand emerges as H20. Under this limitation, remember-ing that every H + signalizes a dissociation, a"working" cross section can be recognized throughthe relation

//70 = \-e-odnx (i)which in this application takes the form

J(H+)/[/(H+) +/(H2+)] = 1 - e-dn* (a)

where the /(H+) and /(Нг+) represent emerging ioncurrents, x is the length of the gas cell, and n is thegas particle density therein. Such a treatment alsoassumes that the dissociations proceed in the simple

DISSOCIATION OF DIATOMIC HYDROGEN IONS 399

о

JI

PRESENT MEASUREflFEDORENKOOAMODARAN

4ENTS

-6ERJU0Y ( H 2 * + H + )

r»—«

A

'•

A- — -

i—' —

120 160ENERGY (kev)

Figure 1. Dissociation cross section of H 2 + in hydrogen gas

fashion H 2

+ + M->H++H° + M. Actually, some mayproceed according to the reaction H2++M —> H+ + H+-b e + M. This process is assumed to occur infrequently;if, however, it were to equal the first process in pro-bability, then the assigned dissociation cross sectionswould have to be reduced by 20 to 40%.

Experimentally, it was easy to measure nx by theattenuation of a proton beam sent through the gascell, knowing the electron capture cross section fromprevious work. Without changing the pressure in thegas cell, the incident ions were then switched to Нг+,and the transmitted beams were measured. The exitaperture of the gas cell and the detector aperture weremade sufficiently large to accept most of the particlesscattered by gas collisions or by the dissociationreaction. In addition, the particle density in the gascell was sufficiently low to prevent multiple collisions.The results for hydrogen as a target gas are shown inFig. 1, where the cross section for dissociation in unitsof square centimeters per gas atom is plotted as afunction of the particle energy. It is seen that the crosssection attains a maximum of 5.7 x 10~17 cm2 per gasatom at 150 kev. Shown also are the experimentalresults obtained by Federenko8 and Damodaran9

which pertain also to ' 'working" cross section in thesense used above, and for comparison the theoreticalresults of Gerjuoy6 are indicated also, despite the factthat they are absolute in character and pertain to adifferent type of dissociation collision. As is seen, allof these results are considerably greater than thecurrently determined values. In Fig. 2 are shown theresults for a target gas of argon. The cross sectiondoes not attain a maximum, but is still increasing atenergies of 220 kev. Again the experimental results ofFederenko and Damodaran are much higher. At thepresent time, the reason for the large discrepancies isunknown. Considerable work has been done to dis-

>y

y_

д

'

- О FtA D

IDORENKO A&M0DARAN

- i

О 40 80 120 460 200 240ENERGY (kev)

Figure 2. Dissociation cross section of H 2

+ in argon gas

tinguish between fast H20 and H° particles by usinga scintillator crystal and determining the pulse heightdistribution of the pulses from a phototube. This willprovide means of accurately measuring the crosssection at the lower energies.

TO 3 Mev VAN OE GRAAFF

7 M C F - 3 0 0 PUMP

V2 X V i e ' " 1 DIA

PRESSURE GAUGES

GAS INLET

8Y-PASS VALVE

МСР-ЗООРиМрС

- */2 X V<6-m ÛIA

ELECTROSTATIC ANALYZER

'*•» NO i D E T E C T O R ^ ^ 0 DETECTOR

Figure 3. Apparatus used to measure dissociation crossjection athigh energies

400 SESSION A-10 P/1789 С F. BARNETT

ENERGY (kev)

500 750 Ю00 1250 1500 1750 2000 2250

~ 2

10H

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80

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ENERGY (kev)750 1000 1250 1500 1750

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2250

1

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8 Ю 12 14 1 6 ( x 1 0 8 ) U 4 5 6 7 8 9 10 11 12 13 14 15(X108)

VELOCITY (cm/sec) VELOCITY (cm/sec)

Figure 5. Percent of dissociation by reactionH2++M -> H++ Ho+M

Figure 4. Dissociation cross section, per atom of gas traversed,as a function of particle velocity for various gases

At energies above 500 kev the measurementsbecome significant in an absolute sense, because of thereduction in the electron capture cross section for H2+.Figure 3 is a schematic diagram of the apparatus usedin this energy region. The ion beam from a 3 Mv Vande Graaff accelerator was incident upon a differentially-pumped gas cell in which the pressure was sufficientlylow to prevent multiple collisions. This pressure wasdetermined by measuring the ratio of H° to H+ in thedissociated beam. At pressures conducive to multiplecollisions, the H°/H+ ratio decreased as a result of theease with which the hydrogen neutral particles lose anelectron. Emerging from the gas cell was a mixture ofH°, H+, and H2+. Using the set of electrostatic deflec-tion plates, the beam was analyzed into its variouscomponents. The zero detector was used as a secondaryelectron detector to measure both the neutral com-ponent and the charged component of the beam. TheNumber 1 detector was used as both a secondaryelectron detector and a Faraday cup. The secondaryelectron emission produced by H° from a brass targetwas corrected for the difference in the mean number ofsecondary electrons emitted by neutral particle impactas compared with proton impact. This correction wasdetermined by measuring the rise in temperature ofthe target with only ions or atoms impinging, whileat the same time measuring the secondary electronemission. The ratio of the relative secondary electronemission for hydrogen atoms and ions increased from1.36 at the lowest energy to 1.59 at the highest energy.

The total beam available for dissociation was foundby summing the emergent H24" with the number ofreactions occurring, found by particle balance betweenH° and H+. Using the same particle or charge balance,one can also determine the fraction of the reactionsgoing by simple dissociation (i.e., H 2

+ +M->H+ +H° + M), or an ionizing dissociation (i.e., H2+ + M->2H+ + e + M). From these measurements one deter-mined the cross section from Eq. (1), in which / is equalto the total number of reactions and /0 equals the totalНг+ available for dissociation.

To determine the extent to which scattering mightbe influencing the measured cross section, the exit pinhole was enlarged. The results indicated that the crosssection is independent of the geometry used. Gaspressure was measured with an accurately calibratedMcLeod gauge. The path length for dissociation wastaken as the geometric length of the gas cell. The usualprecautions were taken in regard to the purity of thetarget gases. Background gas and aperture collisionsgave a background of less than 1% which was sub-tracted from the collisions occurring in the gas cell.It is estimated that the accuracy of the results shouldbe within ± 15%.

The results obtained are shown in Fig. 4. The crosssection is plotted as a function of the particle energyfor target gases of hydrogen, helium, nitrogen, andargon. For hydrogen and helium, there is an approxi-mate v1 dependence. For nitrogen and argon, thevelocity dependence is less marked, the cross sectiondecreasing only 15 to 20% over the entire energyrange. Shown also are the theoretical results derivedby Salpeter5 for nitrogen. The predicted theoreticalvalues and the experimental values seem to be inagreement within a factor of two; however, thevelocity dependence is seen to be quite different.Shown in Fig. 5 is the fraction of the reactions pro-ceeding by simple dissociation (as contrasted with ion-izing dissociation) for hydrogen and argon. The modeof dissociation apparently is dependent only on thetarget gas, and is independent of the particle energy.

DISSOCIATION IN A VACUUM CARBON ARC

The measurements described above indicated thatthe cross section for dissociation of the diatomichydrogen ion by impact with a gas atom is a slowlyvarying function of the particle velocity. In a searchfor an efficient medium for dissociation, Luce1» 2 hasused a carbon arc in a magnetic field. The arc is a dis-charge of hundreds of amperes operating in a vacuumregion at a pressure of about 10~5 mm mercury. Pre-

DISSOCIATION OF DIATOMIC HYDROGEN IONS 401

vious reports4 gave a dissociation efficiency of 40%at an energy of 20 kev. Since no adequate explanationhad been advanced for the high dissociation efficiency,it was difficult to predict the dependence upon energy.Accordingly, an experiment was performed to deter-mine the dissociation efficiency at energies ~ 600 kev.

If it is assumed that dissociation of the diatomicions in the carbon arc is the result of particle inter-action, then one may write the following as probablereactions leading to dissociation:

(2)

(3)

(4)

(5)

(6)

2H+ + e + O

H2++ e

H2+ + e

where n may vary from 0 to the highest state ofionization in the arc. The first two reactions and thelast two involve electron excitation, while reaction (4)is a charge exchange reaction with the carbon ioncapturing the electron from the diatomic ion. Thepresent experiment cannot distinguish between re-actions (2) and (5) or between (3), (4) and (6). However,by using a charge balance between H° and H+, onecan determine the fraction of the reaction proceedingby (2) plus (5) and the fraction proceeding by (3) plus(4) plus (6). For simplicity, the dissociation efficiencywill be denned as the ratio of the H+ current to thesum of the H+ and the H2+ current. A schematicdiagram of the apparatus is shown in Fig. 6. The ionswere formed in a conventional radiofrequency ionsource and accelerated to 600 kev by the ORNLcascade accelerator. Mass analysis was accomplishedby means of a 90° magnetic analyzer. The analyzedion beam was defined by a movable |-inch apertureplaced directly in front of the solenoid containingthe carbon arc chamber. On the exit side of the arcchamber was a fan-shaped container in which anion detector could be moved vertically to measurethe various emerging beams. The backing plate of the

-JON SOURCE

0.125- in . DEFINING SLIT

4 - i n . ID SOLENOID

TOELECTROMETER

chamber consisted of quartz windows so that thevarious beams could be observed visually. The initialbeam was analyzed into five separate beams by passingthrough the solenoid. In the region between the solen-oid and the analyzing magnet a fraction of the dia-tomic ions was dissociated into protons and neutralatoms. The protons from the dissociation were de-flected through a large angle by the solenoid field,whereas the neutral particles traveled in an un-deflected trajectory. In addition to these two beams,there was also a proton and neutral particle beamresulting from dissociation in the arc. This neutralbeam was separated from the other neutral beam,because the H2+ was deflected by the magnetic fieldbefore suffering a dissociation collision in the arc.There was also present a fraction of the initial beamof H2+. The dissociation process was found to takeplace in an extended region of the arc, so that protonsand atoms were formed at different positions in themagnetic field. This resulted in a vertical spread ofboth the proton and neutral beams of approximatelyone inch, which was larger than the detector aperture.In measuring ion beam intensities, the detector wasmoved at a constant linear rate across each beam by asynchronous motor. The beam current was measuredby an electrometer whose output was plotted by aBrown recorder. The area under the recorder tracewas integrated with a planimeter to determine beamintensity. The procedure consisted in measuring inturn H2+, H+, H°, and H2+. When the two H2+ re-cordings differed more than 10%, the run was dis-carded.

A more detailed diagram of the solenoid and arcchamber is shown in Fig. 7. The chamber was anevacuated stainless steel tube, 6 ft. long and 4 in. indiameter. Surrounding this tube was the magneticcoil consisting of 7 layers of water-cooled coppertubing, each layer having 3.2 turns per in. The coilwas separated in the center to provide a gap of 2 in.for the beam entrance and exit. Inside the tube wasa 3 in. diameter water-cooled copper liner to dissipatethe power radiated from the arc. The arc electrodeswere Union Carbide C-18 grade graphite, pressed intowater-cooled copper sleeves. Quartz tubing was placedaround the cathode assembly to prevent arc-over inthe fringing magnetic field region. Various anode con-figurations were used, and these will be discussedbelow. The anode and cathode assemblies weremounted so that the arc length could be variedbetween 6 in. and 4 ft.

WATER-COOLEDCOPPER L INER-

BEAM IN

CATHODE-12 in.

Figure 6. Apparatus used in determining dissociation efficiencyof carbon arc Figure 7. Schematic diagram of arc chamber

402 SESSION A-10 P/1789 С F. BARNETT

•s 4 0 °ï

ARC LENGTH (in )

Figure 8. Carbon arc voltage as a function of arc length

12

10

Arb

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о Radiât

1

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10 12 14Distance from anode (in )

Figure 9. Н г + dissociation and energy radiated from the arc as afunction of the distance from the anode

450 470 490 240 230 250 270 290 340 330 350178» ARC CURRENT (amperes)

Figure 10. H+ current resulting from dissociation in the carbon arcas a function of the measured arc current

The detection of the energetic particles was com-plicated by high radiofrequency fields produced bythe arc and also by a copious supply of photoelectronsejected from the detector and from walls surroundingthe detector region by the intense ultra-violet radia-tion from the arc. The detector found most usefulconsisted of a Faraday cup and a bias suppressor ringmounted inside a completely shielded box. The beamsentered through a 50 /xin. nickel foil soldered overa f-in. aperture. The foil was sufficiently thickto insure an equilibrium charge distribution betweenenergetic positive ions and neutral particles emergingfrom the back surface. The characteristic charge dis-tribution was independent of the initial charge stateand depended only on the particle energy and foilthickness. The incident Нг+ beam was immediatelydissociated at the surface of the foil resulting in twoparticles of half energy, each of which registered withthe same efficiency as the H° and H+.

The procedure used in striking a carbon arc was theone customarily used with shorter dc arcs.2 To thearc electrodes were applied 350 v dc (open circuitvoltage of the four series connected welding generatorswhich supplied the arc power) and a comparable

rf voltage. When gas was admitted through a passagein the cathode, a radiofrequency arc appeared, fol-lowed immediately by the main discharge. The rfvoltage and the gas input were removed as soon as thedc arc had fired. Thereafter, the arc was supportedsolely by carbon vaporized from the electrodes. Threehundred amperes was the customary arc current andvoltages ranged from 70 to 100 v depending uponthe arc length. The voltage relationship is shown inFig. 8, in which the slope of the curve is found to be1.0 v/in.

For a 600-kev Щ4" particle, the average dissociationefficiency obtained was 10.6( + 2.1, -0.9)%f whichmay be compared to an average efficiency of 16.4(+1.6, -1.2)% for a D 2

+ particle of the samekinetic energy. These efficiencies were measured at adistance of 6 in. from the arc anode. The arc con-ditions were: (1) 300-amp arc current, (2) |-in.diameter cathode, and (3) f-in. diameter anode.

Attempts to measure the velocity dependence ofthe efficiency with an Нг+ beam were unsuccessfulbecause of difficulties encountered with the detectorat lower particle energies. If it is assumed that thedeuterium ion and hydrogen ion have the sameinelastic collision cross section at identical velocities,then the above values are consistent with a 1/vdependence for the efficiency. It is not readily possibleto compare these figures with those measured at lowerenergies in another geometry because the distancefrom the anode is a critical factor, as will be seen.

By making a charge or particle balance between thedissociated beams of H° and H + and the undissociatedH24", it is possible to determine a fraction of the re-action proceeding by simple dissociation or by anionizing dissociation plus charge exchange. For 600kev H 2

+ particles, the fraction going as simple dis-sociation was found to be 80%. For a deuteriumparticle, this fraction decreased to 65%. In the presentexperiment there could not be excluded the possibilityof the ionization in the arc of some of the H° producedby dissociation in the arc.

In a magnetic containment device, it is desirablethat the dissociated H+ particle, with its multipletraversais of the arc, does not capture an electronfrom either the free electrons or the charged or neutralcomponents of the arc. To determine the probabilityfor electron capture, a 300-kev proton beam was passedthrough the arc and the appropriate region wasscanned for the presence of neutral atoms. None werefound, and the upper limit for the probability ofcapture was estimated at 10~5. The limitations ofsensitivity were determined by the detector and thenoise in the detector circuit.

Numerous changes were made in the geometry ofthe electrodes to determine the optimum conditionsfor dissociation. Initially a cathode | in. in diameterand 10 in. long was used with ¿ in. longitudinalbore to admit gas for striking the arc. The anodewas 1J in. in diameter and 1J in. long. Visual

t The plus and minus figures give the range of deviationsabove and below the average shown.

DISSOCIATION OF DIATOMIC HYDROGEN IONS 403

observation showed the arc to be diverging to a dia-meter of approximately 1 in. in the center of thesolenoid. This swelling was a result of the divergenceof the magnetic field at the 2-in. gap in thesolenoid windings, which had to be provided to permitentrance and exit of the particle beam. The visualobservation of the swelling in the arc was confirmed byplacing a carbon disc in the center of the solenoid andmeasuring the diameter of the hole burned through it.The swelling was undesirable because it would beexpected to reduce the degree of dissociation if thelatter were due to the product of particle density inthe arc and diametrical path length. Therefore aneffort was made to concentrate the arc by using moreslender electrodes; a decrease in cathode diameter to| in. did in fact reduce the arc diameter to f in. atthe point of beam intersection. Attempts further todecrease the diameter of the cathode to | in. wereunsuccessful, because the cathodes shattered whencurrent was passed through them. Previous experi-ments2 had indicated that the efficiency for dissocia-tion at low particle energy is dependent on thetemperature of the anode. By decreasing the anodediameter to § in., the radiant heat was decreasedwith a gain of a factor of 1.3 in the dissociationefficiency.

The efficiency of dissociation was found to be de-pendent upon the distance between the anode and thepoint of intersection of the Нг+ beam. Figure 9 showsthe efficiency as a function of the distance to the anode(for a l|-in. anode diameter). A movement of theanode from a position 3 in. from the H24" beamto 21 in. caused a decrease in the efficiency of dis-sociation by a factor of two. Also, shown in Fig. 9 isthe radiant energy measured at 90° from the arccolumn as a function of the anode distance. This wasmeasured by a thermal detector placed in the sameposition as the particle detector. The thermal detectorconsisted of a copper ring with a 25 juin, nickel foilsoldered to the front surface. Soldered to the center ofthe nickel foil was a fine copper wire, which formed athermocouple indicator with low heat loss, short timeconstant, and sensitivity of the order of microwatts.Glass, quartz, and fluorite of thickness 1 to 3 mm werefound to absorb more than 90% of the radiant energyemitted by the arc. It will be noticed in Fig. 9 that theslope of the radiant energy curve is similar to the slopeof the dissociation efficiency. Both the efficiency andthe radiant energy were independent of the cathodeposition. It may be inferred from the radiant energy

curve either that the density of radiating particles isincreasing or that the level of excitation of the particlesis increasing as regions closer to the anode areexamined.

Figure 10 gives a plot of the dissociated H + currentas a function of the measured arc current. The arccurrent was varied from 160 to 325 amp with acorresponding 1.3-fold increase in the H + current.

Attempts to measure the attenuation of variousbeams by measuring current with and without thearc were unsuccessful because of the pumping actionof the carbon arc. The pressure in the solenoid wasreduced by a factor of 3 with the carbon arc operat-ing, following an initial outgassing period. The lowestpressure obtained in the vacuum system was 4 x 10~6

mm. Hg, as measured by a VG1A Ionization Gaugeplaced 8 in. from the arc along the beam entrancetube. Experiments are continuing on the dissociationproduced by the carbon arc at lower particle energies.Also, energetic particles of various kinds are beingstudied to learn more about the nature of the processestaking place.

CONCLUSION

The measurements obtained in these experimentsserve to show that the probability of dissociation ofH24" ions on passing through a vacuum carbon arcremains sufficiently large to be of usefulness in a trap-ping device when the energy of the incident ions israised as high as 600 kev. The trapped H+ (or D+)beam would apparently neutralize itself to only a smallextent as it circulates through the carbon arc in anexperimental device. Significant additions have beenmade to cross section information concerning the dis-sociation of H2+ ions by collisions in several gases.

ACKNOWLEDGEMENTS

It is a pleasure to acknowledge the help and com-ments of various members of the Oak Ridge NationalLaboratory staff. Acknowledgement is extended toJ. A. Ray and R. G. Reinhardt for help in making themeasurements and to R. J. Mackin Jr., for his helpand comments in making the measurements in PartII. Special acknowledgement is made to J. S. Luce forsuggesting the work and his help in leading the designgroup of H. С Hoy, G. F. Leichsenring, and R. L.Knight in designing the arc assembly and solenoid.The many helpful comments and suggestions of E. D.Shipley, A. H. Snell, P. R. Bell, С D. Moak, and R. A.Dandi are greatly appreciated.

REFERENCES

1. J. S. Luce, Proposed Sherwood Experiment, Oak RidgeNational Laboratory memo, ORNL CF-55-4-73 (1955).

2. J. S. Luce, Status Report on Carbon Arc, Oak RidgeNational Laboratory report, ORNL-2219 (1957).

3. P. B. Moon, CERN Symposium on High Energy Accelera-tors and Pion Physics, Vol. 1, 231-234, Geneva (June11-23, 1956).

4. J. S. Luce, Studies of Intense Gas Discharges, P/1790, Vol.31, these Proceedings.

5. E. E. Salpeter, Dissociation Cross Sections for Fast Hydro-gen Molecule Ions, Proc. Phys. Soc. (London), A63, 1295-1297 (1950).

6. E. Gerjuoy, Dissociation and Ionization of Я24" by FastProtons, Research Report 60-94439-1-R2, WestinghouseResearch Laboratories, Pittsburgh, Pennsylvania.

7. К. Е. A. Efíat, Dissociation of Molecule Hydrogen Ions inthe Cyclotron, Proc. Phys. Soc. (London), A65, 433-436(1952).

404 SESSION A-10 P/1789 С F. BARNETT

8. N. V. Federenko, Magnetic Analysis of a Positive-IonBeam Accelerated гп a ô-30-kv. Field, After PassageThrough Rarefied Gas, Zhur. Tekh. Fiz., 24, 769-783(1954).

9. К. К. Damodarçn, Dissociation of Molecule Hydrogen Ions(Я2+) in Gases, Pçoc. Roy. Soc. (London) 239A, 382-393(1957).

10. P. M. Stier, С F. Baroçtt and G. E. Evans, Charge States

of Heavy-I on Beams Passing Through Gases, Phys. Rev.,96, 973-982 (1954).

11. P. M. Stier and С F. Barnett, Charge Exchange CrossSections of Hydrogen Ions in Gases, Phys. Rev., 103, 896-907 (1956).

12. C. F. Barnett and H. K. Reynolds, Charge Exchange CrossSéchons of Hydrogen Particles in Gases at High Energies,Phys. Rev., 109, 355-359 (1958).